A capacitor includes two electrodes: an anode and a cathode. As a material of the anode, there is used a valve metal such as aluminum or tantalum which allows an insulating oxide film to be generated on a surface. As a material of the cathode, there is used any of an electrolytic solution, an inorganic semiconductor, an organic conductive material and a metal thin film. When the material of the cathode is an electrolytic solution, an aluminum foil having an enlarged surface area is frequently used for a cathode terminal. An aluminum foil of this type is called an electrolytic capacitor cathode aluminum foil. The surface area of the cathode aluminum foil is enlarged to increase the capacitance of a capacitor.
A manufacturing process of a capacitor involves a step of cutting a cathode aluminum foil supplied in the form of a coil into a predetermined width, a step of coiling the slit coil-shaped cathode aluminum foil which has been cut, and a step of coiling the cathode aluminum foil in order to finally constitute a cylindrical capacitor or a step of cutting the cathode aluminum foil in order to constitute a laminate-type capacitor.
In order to improve productivity in these steps, it is required to raise the rate of coiling the cathode aluminum foil. To meet this demand, it is necessary to improve the mechanical strength of the cathode aluminum foil, that is, the tensile strength of the cathode aluminum foil.
As mentioned above, the cathode aluminum foil requires a high tensile strength as well as a high capacitance as necessary requirements.
For increasing the capacitance of the cathode aluminum foil, there is generally adopted a method of enlarging the surface area of the aluminum foil by etching. The etching, however, significantly reduces the tensile strength of the aluminum foil.
There is a current need for developing a smaller capacitor along with the development of small-sized electric apparatuses. To make a capacitor small, the thickness of the cathode aluminum foil must be made more small. If the thickness of the aluminum foil is made thin, the strength of the aluminum foil is relatively dropped.
Also, as the capacitor cathode aluminum foil, one having an aluminum purity of about 99.0 to 99.95% by mass is usually used to control the reaction when carrying out etching to enlarge the surface area of the aluminum foil. The purity (base purity) of aluminum herein means a value obtained by subtracting the content of major three elements, i.e., iron, silicon and copper, included in the aluminum foil from 100%.
In this situation, there is proposed a manufacturing method for improving the capacitance and strength of an electrolytic capacitor cathode aluminum foil in, for example, Japanese Unexamined Patent Publication No. 05-247609. In this manufacturing method of the electrolytic capacitor cathode aluminum foil, an aluminum material having a chemical composition in which the purity of aluminum is 99.8% or more, the contents of Fe and Si are respectively limited to 0.05% or less and the content of Cu is limited to 0.005% or less and the content of Mg and the content of Zn are adjusted so as to satisfy a predetermined equation, is subjected to soaking treatment at 540° C. or less, hot-rolled and then subjected to coiling which is finished at a temperature of 300° C. or less, and is finally subjected to cold rolling carried out at a working ratio of 95% or more to make the product foil have the specified thickness. This aluminum foil is etched to enlarge the surface area.
However, even if the foregoing manufacturing method disclosed in Japanese Unexamined Patent Publication No. 05-247609 is used, it is limited in its capability of improving the capacitance of a capacitor without any reduction in tensile strength as to the requirements satisfied by the capacitor cathode aluminum foil.